This enzyme, along with EC 2.3.1.38, [acyl-carrier-protein] S-acetyltransferase, is essential for the initiation of fatty-acid biosynthesis in bacteria. This enzyme also provides the malonyl groups for polyketide biosynthesis [7]. The product of the reaction, malonyl-ACP, is an elongation substrate in fatty-acid biosynthesis. In Mycobacterium tuberculosis, holo-ACP (the product of EC 2.7.8.7, holo-[acyl-carrier-protein] synthase) is the preferred substrate [5]. This enzyme also forms part of the multienzyme complexes EC 4.1.1.88 (biotin-independent malonate decarboxylase) and EC 4.1.1.89 (biotin-dependent malonate decarboxylase). Malonylation of ACP is immediately followed by decarboxylation within the malonate-decarboxylase complex to yield acetyl-ACP, the catalytically active species of the decarboxylase [12]. In the enzyme from Klebsiella pneumoniae, methylmalonyl-CoA can also act as a substrate but acetyl-CoA cannot [10] whereas the enzyme from Pseudomonas putida can use both as substrates [11]. The ACP subunit found in fatty-acid biosynthesis contains a pantetheine-4'-phosphate prosthetic group; that from malonate decarboxylase also contains pantetheine-4'-phosphate but in the form of a 2'-(5-triphosphoribosyl)-3'-dephospho-CoA prosthetic group.